Cut the onion in half and stick a clove into the round part of each half.
Next, create a bouquet garni by tying together the bay leaf, parsley, and basil.
Place the celery, carrot, the zucchini, the two onion halves, garlic, and the tomato pierced with a fork into a large pot, then cover with the indicated amount of cold water.
Heat the pot over a low temperature between 37°C and 67°C (99–153°F) to ensure maximum flavor extraction (a thermometer can be helpful). Temperatures up to 95°C (203°F) can also be suitable, as long as the water does not boil.
Cook the broth for about 2 hours, skimming off any foam that forms on the surface from time to time.
After the cooking time, strain the broth first through a fine-mesh sieve and then through cheesecloth or muslin.
Finally, adjust with salt and pepper to taste and serve hot.